Nitrogen deposition and herbivory affect biomass production and allocation in an annual plant

Abstract

Atmospheric nitrogen (N) deposition is leading to large‐scale changes in N availability in terrestrial ecosystems. Nitrogen availability is known to affect herbivore consumption rates, insect population dynamics and plant response to herbivory, but few studies have investigated these factors in combination. I tested the hypotheses that a) simulated N deposition alone would positively affect plant performance, b) herbivory alone would detrimentally affect plant performance and c) when combined, the positive impacts of N deposition would be mitigated by N‐induced increases in herbivore consumption rates and population sizes. I assessed the response of Ambrosia artemisiifolia (Asteraceae, common ragweed) to simulated N deposition and herbivory by aphids and beetles. In greenhouse and field studies, I caged multivoltine herbivores on plants and allowed damage rates to differ among N treatments due to N‐driven changes in herbivore populations and consumption rates. Simulated N deposition led to increased vegetative and seed biomass and decreased root:shoot ratios. Both types of herbivory caused decreases in seed biomass, reproductive allocation, and root biomass. The combined influences of N deposition and herbivory varied with plant tissue type. The positive influences of N deposition on root and seed biomass were generally suppressed by herbivory, while positive impacts of N on shoot biomass were typically not significantly suppressed by herbivory. These data suggest that in addition to substantial direct influences on plant production and allocation, N deposition may indirectly affect biomass production and allocation through affecting the rates and consequences of insect herbivory. However, these strength of these indirect effects will differ among herbivore species. The particularly strong influence of both herbivory and N deposition on A. artemisiifolia reproduction suggest potential population and community‐level consequences.